1. Field of the Invention
The present invention relates to polymeric blends. In particular the invention relates to methods of producing melt-processable multiphase thermoplastic compositions comprising a crystalline polyolefin resin and a cross-linked polyacrylate containing chemically functional groups.
2. Description of Related Art
Blends of plastics and elastomers are well-known in the art. Since the polymer phases of such blends are generally not compatible with each other, the resulting conventional compositions have poor physical properties. Therefore, blends of non-polar plastics and polar elastomers, such as those disclosed in U.S. Pat. Nos. 4,555,546 (granted Nov. 26, 1985 to Monsanto), and 4,782,110 (granted Nov. 1, 1988 to DuPont), normally require incorporation of a compatibilizing agent for obtaining satisfactory physical properties. The addition of such compatibilizing agents, however, adds to the expense of the blend and complicates the manufacturing process. Certain compatibilizing agents and curatives will also contribute colour to the composition.
Furthermore, when the elastomer component of blends of the above kind is uncured, the blend has high compression set and high oil swell.
An additional problem related to the multiphase thermoplastic compositions described in the above patents resides in the fact that the most desirable polar elastomers are only available in the form of large sticky chunks rather than the free-flowing pellets used for feeding conventional compounding extrusion equipment. As a result, the process for manufacturing these multiphase thermoplastic compositions uses expensive batch mixing rubber compounding equipment. Alternatively, an expensive cryogenic grinding process has to be employed to convert the sticky chunks of raw rubber to a fine powder that can be made free-flowing by the addition of a partitioning agent.
In addition to compatibilizers, crosslinking agents (other than peroxides) have also been used in attempts to crosslink the polyacrylate phase by reacting the functionality of an ethylene/acrylate copolymer with a diamine (U.S. Pat. No. 4,782,110). So far these agents appear to be rather ineffective.
In addition to the above-mentioned traditional blends, known in the art are also single free-flowing pellets consisting of melt-processible multiphase thermoplastic compositions made up of intimate homogeneous blends of polyolefins and polyacrylate elastomers. According to the teaching of U.S. Pat. No. 5,300,578, these blends can be produced by polymerizing liquid monomeric units of the acrylate component into solid elastomeric polar polymer within the pellet structure of the polyolefin component. This free radical polymerization process occurs at relatively low temperature and under low mechanical shear conditions to provide an intimate homogeneous mixture of the components in the form of a free-flowing pellet.
Although compositions of the last-mentioned kind represent a clear improvement over the prior art, tensile strength, elongation and, in particular, compression set resistance of these known blends are still unsatisfactory.